This invention relates to combined cycle power generation systems, and, more particularly, to such a system which incorporates a carbonizer to produce a low-Btu fuel gas which is combusted in a topping combustor to produce hot exhaust gas for the operation of a gas turbine.
Combined cycle power generation systems are well known in the art and typically involve the combustion of natural gas or oil under pressure for the generation of hot gases which are passed through a gas turbine where the gases expand and cool while performing work in the generation of electrical power. The turbine exhaust gases are passed to a heat recovery unit for the generation of high temperature steam which is used by steam turbines to perform additional work.
Combined cycle power generation systems typically have relatively high efficiency because the steam turbines operate at substantially lower temperatures than the gas turbine. Combined cycle systems, unfortunately, also require the use of premium fuels, such as natural gas or oil, for the operation of the gas turbine and are therefore considered too expensive for many industrial operations.
To increase the system efficiency and to lower the operational cost of combined cycle power generation systems, pressurized fluidized bed reactors have been incorporated in which a fluidized bed, burning a low cost fuel such as coal, is operated under a pressure of between approximately 10 to 15 atmospheres. The flue gases from the bed are passed through a cyclone separator and a ceramic cross-flow filter that operate to separate the entrained solids from the gases. The solids are returned to the reactor bed and the clean gases are passed through a gas turbine where energy is extracted as the gases cool and expand, before the gases are used to generate steam. A combined cycle system of this sort has a relatively high overall efficiency when compared to similar systems.
Unfortunately, pressurized circulating fluidized bed reactors and gas turbines have conflicting operational requirements for efficient system operation. For example, a gas turbine requires a relatively high-volume of high-temperature gases for efficient operation; however, a pressurized fluidized bed reactor burning a reactive fuel should be operated with a lower-volume of combustion supporting gas, to maintain the fuel particles at a temperature close to the gas and other solids temperature, for efficient absorption of SOx, to prevent the emission of NOx and alkaline gases, and to prevent agglomeration of particulate material. Consequently, it is difficult to integrate the operational requirements of a circulating pressurized fluidized bed reactor with those of a gas turbine and maintain a high efficiency, meet emission requirements and avoid agglomeration of particles in the fluidized bed reactor.